In recent years considerable evidence has accumulated which suggests that extra-geniculo-striate mechanisms are involved in visual information processing. This has led to the concept of secondary visual systems in which the superior colliculus has been suggested to play an important role. Behavioral findings in the golden hamster have allowed the most clear-cut differentiation between the visual functions which might be subserved by cortical mechanisms and those which may be mediated by midbrain structures. The primary objectives of the proposed research are: (1) to delineate the response properties of collicular neurons in the hamster and (2) to determine the possible contribution of cortical influences to these responses. These objectives will be achieved in two types of experiments. First, the responses of collicular units in intact, acutely prepared, adult hamsters will be investigated with special regard to the vertical organization of the visual, auditory and somatosensory neurons. Second, the role of the cortical contribution to the response properties of tectal units will be examined both by acute lesions of cortex and also through reversible cortical cooling. The results of these studies will provide information about the neurophysiological properties of the superior colliculus in the animal whose visual behavior has provided a major impetus for the "two visual systems" hypothesis.